1. Field of the Invention
The present invention relates to a variable output voltage regulator including an overcurrent protection circuit.
2. Description of the Related Art
Description is made of a conventional variable output voltage regulator. FIG. 10 is a diagram illustrating the conventional variable output voltage regulator.
The conventional variable output voltage regulator includes a ground terminal 100, a power supply terminal 101, an output terminal 102, an external control terminal 103, an amplifier 104, an output transistor 105, a voltage dividing circuit 106, a reference voltage source 107, a drooping type overcurrent protection circuit 108, a fold-back type overcurrent protection circuit 109, and a trimming signal generation circuit 110.
Description is made of an operation of the conventional variable output voltage regulator.
When an output voltage Vout of the output terminal 102 is higher than a predetermined voltage, that is, when a divided voltage Vfb of the voltage dividing circuit 106 is higher than a reference voltage Vref, an output voltage of the amplifier 104 becomes higher. A gate voltage of the output transistor 105 increases, and hence the output transistor 105 is gradually turned OFF and the output voltage Vout decreases. On the other hand, when the output voltage Vout is lower than the predetermined voltage, the output voltage Vout increases in the same manner as described above. In other words, the output voltage Vout of the voltage regulator is maintained to a constant predetermined voltage.
In this case, signals φ1, φ2, and φ3 output from the trimming signal generation circuit 110 in accordance with an electrical signal CONT input from the external control terminal 103 are input to gates of MOS switches connected in parallel to resistors 151, 152, and 153 in the voltage dividing circuit 106, respectively. Therefore, based on the electrical signal CONT, a voltage division ratio of the voltage dividing circuit 106 can be adjusted.
The output voltage Vout of the voltage regulator is determined based on the reference voltage Vref and the voltage division ratio of the voltage dividing circuit 106. Therefore, the output voltage Vout can be controlled by a signal input to the external control terminal 103. In FIG. 10, the variable output voltage regulator is realized by the switches connected in parallel to the respective resistors 151, 152, and 153, but the numbers of resistors and switches, the type of the switches, and positions of the resistors connected to the switches are not limited thereto (see Japanese Patent Application Laid-open No. 2005-293067).
Next, description is made of an operation of the conventional overcurrent protection circuit of the voltage regulator.
The overcurrent protection circuit is divided into a drooping type overcurrent protection circuit and a fold-back type overcurrent protection circuit. In the conventional overcurrent protection circuit, the drooping type overcurrent protection circuit and the fold-back type overcurrent protection circuit both operate so as to detect an output current Iout flowing through the output transistor 105, and control the gate voltage of the output transistor 105 to prevent an output current equal to or larger than a certain amount from flowing therethrough.
FIG. 11 is a graph showing output voltage-output current characteristics in a case where the drooping type overcurrent protection circuit and the fold-back type overcurrent protection circuit are used in combination. The drooping type overcurrent protection circuit has the following feature. In order to limit the output current Iout to a constant maximum current Im, when a current equal to or larger than the maximum current Im is caused to flow, the drooping type overcurrent protection circuit decreases the output voltage Vout while maintaining the constant output current Iout. In this manner, the heat loss which occurs in a load of the voltage regulator can be reduced. On the other hand, the fold-back type overcurrent protection circuit reduces the output current lout in proportion to the reduction of the output voltage Vout when the output voltage Vout becomes equal to or smaller than a limiting voltage Vfo. That is, when the output voltage Vout is 0 V, the output current Tout is fixed to a constant short-circuit current Is. The fold-back type overcurrent protection circuit can further reduce the heat loss which occurs in the voltage regulator. In this case, the maximum current Im, the short-circuit current Is, and the limiting voltage Vfo are preset in the circuit (see Japanese Patent Application Laid-open No. 2005-293067).
However, in the conventional variable output voltage regulator including the overcurrent protection circuit, when the maximum output voltage Vout is set, the loss during the operation of the drooping type overcurrent protection circuit increases.
Power loss P of the voltage regulator is represented as follows:P=(Vin−Vout)×Iout  (1)where Vin represents an input voltage of the power supply terminal. Thus, the loss becomes the maximum when there is a great voltage difference between the input voltage Vin and the output voltage Vout. That is, the loss becomes the maximum at (Iout, Vout)=(Im, Vfo) in FIG. 11 when the operation of the drooping type overcurrent protection circuit is switched to the operation of the fold-back type overcurrent protection circuit. As the setting value of the output voltage Vout becomes high, the input voltage Vin needs to be high. Therefore, as the setting value of output voltage Vout becomes high, the loss at (Iout, Vout)=(Im, Vfo) increases, which may lead to circuit damage even when the overcurrent protection circuit is operated.